1. Field of the Invention
The present invention relates to a racket frame and more particularly to a racket frame formed by layering fiber reinforced resinous layers one upon another. In the present invention, a matrix resin which impregnates reinforcing fibers of the racket frame therewith is improved to allow the racket frame to be lightweight and have superior vibration-damping performance without deteriorating the strength and rigidity thereof.
2. Description of the Related Art
In playing tennis, vibrations generated when a player hits a tennis ball and shocks applied to the player's hand make the player uncomfortable and are considered to be a cause for tennis elbow. Therefore various devices have been made to suppress vibrations generated when the player hits the tennis ball. In a representative vibration-suppressing method, thermoplastic resins having a high vibration-damping performance are used as the matrix resin of the fiber reinforced resin composing the racket frame.
For example, in the racket proposed by the present applicant and disclosed in Japanese Patent Publication No. 5-33645, the thermoplastic resin consisting of the nylon resin having a high vibration-damping performance is used as the matrix resin. According to the disclosure, the vibration-damping ratio of this racket is about twice as high as that of a racket whose frame contains a thermosetting resin (for example, epoxy resin) as its matrix resin, supposing that the volume ratio of the fiber reinforcing the thermoplastic resin is equal to that of the fiber reinforcing the thermosetting resin.
As disclosed in Japanese Patent Application Laid-Open No. 10-290851, the present applicant also proposed the racket frame composed of the epoxy resinous composition containing the rubber-like polymeric component and the (meta) acrylic polymeric fine particles in a dispersed state. The racket frame has an improved vibration-damping performance without deterioration of its rigidity and strength and a low degree of fluctuations in its vibration-damping performance.
As disclosed in Japanese Patent Publication No. 61-29613, there is disclosed the prepreg containing the rubber-modified epoxy resin, having a sea-island structure, as its matrix resin. The epoxy resin and the liquid rubber compatible with the epoxy resin are hardened by uniformly compatibilizing the epoxy resin and the liquid rubber with each other.
As disclosed in Japanese Patent Application Laid-Open No. 2002-45444, the present applicant also proposed a racket frame having the viscoelastic material disposed at one or more positions of the fiber reinforced resinous layer thereof as the vibration-absorbing material. The viscoelastic material has a loss factor (=tan δ) not less than 1.00 and a thickness not less than 0.1 mm nor more than 0.6 mm, when the loss factor is measured at a temperature of 6° C. and a frequency of 10 Hz.
In the racket disclosed in Japanese Patent Publication No. 5-33645, the reason the nylon resin serving as the matrix resin has excellent vibration-damping performance is because water serves as a plasticizer and the glass transition temperature drops greatly. The glass transition temperature is about 60 degrees in an absolute dry state, but drops as the water absorption increases. Thus the glass transition temperature becomes about 20 degrees in the vicinity of the room temperature when the water absorption becomes 3%. Therefore the vibration-damping ratio of the racket is 0.005 in the absolute dry state, but 0.020 when the water absorption is saturated. That is, when a humidity changes, the performance of the racket changes. Thus although the vibration-damping performance of the racket can be enhanced, there is room for improvement in the degree of dependence on environment and making its weight lightweight.
In the racket frame disclosed in Japanese Patent Application Laid-Open No. 10-290851, although the racket frame has an excellent vibration-damping performance, the epoxy resinous composition containing the rubber-like polymeric component and the (meta) acrylic polymeric fine particles in a dispersed state has a high viscosity. Thus it is frequently difficult to mold the epoxy resinous composition. Further there is still room for improvement in making the weight of the racket frame lightweight and efficient realization of its vibration-damping performance.
Although the prepreg disclosed in Japanese Patent Publication No. 61-29613 has self-adhesion, it has a problem of incapable of enhancing the vibration-damping performance of the racket efficiently by making the racket lightweight and durable.
The rigidity of the racket frame may deteriorate owing to the influence of the viscoelastic material interposed between the adjacent fiber reinforced resins, which leads to the drop of its restitution coefficient. Thus the racket frame is demanded to improve its rigidity, strength, vibration-damping performance, and make it lightweight in a favorable balance.